Method and device for modulating or stabilizing a neutron flux obtained from an ion accelerator

ABSTRACT

A method and device for modulating or stabilizing a neutron flux obtained by bombarding a suitably charged target with nuclear particles which are produced in a pulsed form. The pulsation frequency of the particle source is modulated or stabilized and the duration of the pulsations of said source is controlled in dependence on the measurement of the neutron production rate of the target.

United States Patent Pierson [451 Apr. 18, 1972 [54] METHOD AND DEVICEFOR 561 References Cited MODULATING OR STABILIZING A NEUTRON FLUXOBTAINED FROM AN UMTED STATES PATENTS ION ACCELERATOR 2,867,728 1/1959Pollock ..250/83.l X 3,034,008 5/1962 Soloway ....250/83.l X Inventor:Alexandre Pierson, Blvlers, France 3,176,136 3/1965 Hopkinson ..25o/s4.s

' t A LE At [73] Asslgnee Primary ExaminerArchie R. BorcheltAttorney-Craig, Antonelli and Hill [22] Filed: June 16, 1969 211 App].No.: 833,346 [57] ABSTRACT A method and device for modulating orstabilizing a neutron P D a flux obtained by bombarding a suitablycharged target with [30] Foreign Apphcamn "omy a nuclear particles whichare produced in a pulsed form. The June 28, 1969 France ..157055pulsation frequeney of the particle source is modulated ta.

bilized and the duration of the pulsations of said source is con- [52]U.S.C| ..250/83.1, 250/845 t ll d i d e dence on the measurement of theneutron [51 1 Inl. Cl- I production rate of the target [58] Field ofSearch ..250/83.l, 84.5

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ATTORNEY 5 METHOD AND DEVICE FOR MODULATING OR STABILIZING A NEUTRONFLUX OBTAINED FROM AN ION ACCELERATOR with tritium (titanium orzirconium) by means of a deuteron l0 beam produced by a particleaccelerator neutrons are produced in accordance with the T (d,n) areaction. It is frequently found necessary in studies relating toreactor physics, in nuclear chemistry research or in applicationsrelating to chemical activation analysis to endeavour to modulate theneutron flux thus obtained according to a number of different laws andespecially sinusoidal laws or to stabilize the value of said flux overperiods of time which can amount to several hours.

A common method consists in directly modulating or stabilizing theintensity of the ion beam of the accelerator by controlling the beamextraction voltage according to the desired law.

A second method consists in producing an electrostatic deflection of thebeam, the voltage applied to the deflection plates being directlymodulated or stabilized; the ion beam of constant intensity thusbombards an area of the target which is either variable or constant intime.

However, the effectiveness of these methods is insufficient, especiallyby reason of wear of the target, of lack of homogeneity as well asvariations in neutron yield of the target as a function of thecomposition of the deuteron beam. In fact, the modulation orstabilization of the neutron flux follows the imposed modulation orstabilization with a degree of precision which does not exceed percent.

in order to improve this precision, it has been proposed while retainingthe principle of electrostatic beam deflection to control the modulationor the stabilization of the voltage applied to the deflecting platesaccording to the output voltage of a neutron detection unit. Althoughthis particular method does make it possible to attain a precision of 1percent in respect of frequencies below 100 c/s, substantial errors areintroduced above this value as a result of distortions and fluctuationsin counting. Moreover, it is necessary to place a cooled diaphragm anddeflector plates between the output of the accelerator and the targetbut this is a cumbersome arrangement which gives rise to practicaldifficulties. Moreover, the diaphragm produces stray neutrons and thewear on the target is both irregular and incomplete. Finally, since theposition of the beam on the target is variable, the geometry of theneutron source is not well defined.

The primary object of the present invention is on the one hand, bymaking use of a pulsed ion source whose ignition frequency is modulatedor maintained constant, to permit the use of an electronic controldevice of the digital type which is much more accurate than the deviceof the analog type employed in deflection systems and, on the otherhand, to ensure perfect definition of the geometry of the neutron sourceduring the course of the experiment.

More precisely, the present invention is directed to a method ofmodulation or stabilization of a neutron flux obtained by bombardment ofa suitably charged target by means of nuclear particles, wherein saidparticles are produced in a pulsed form, said method being characterizedin that the pulsation frequency of said source is modulated orstabilized and that the duration of pulsations of the source is madedependent on the measurement of the neutron output of said target.

This invention is also directed to a practical device for carrying outsaid method, said device being characterized in that it comprises apulsed source of nuclear particles,

a neutron-generating target which receives said particles,

a detection unit placed in proximity to said target for the measurementof its neutron output,

a generator for producing either modulated or continuous voltage,

an electronic clock for delivering pulses at variable frequency andcontrolled by said generator,

and an electronic circuit for controlling said source and receivingtrigger pulses from said electronic clock and blocking pulses from saiddetection unit.

Further properties and advantages of the present invention will becomeapparent from the following description in which one form ofconstruction of the device under consideration is given by way ofexplanation but without any limitation, reference being made to theaccompanying drawings, in which FIG. 1 is a general block diagram of thedevice in accordance with the invention, and

FIG. 2 is a diagram of the electronic circuit for controlling thefrequency and duration of bursts of particles with which the target isbombarded,

From FIG. 1, it is seen that a particle accelerator which is showndiagrammatically at 1 bombards with deuterons, for example, a target 2which is formed of metal alloyed with tritium the deuterons producedneutrons at said target according to the Y (d,n) a reaction. It issought to modulate this neutron flux according to a given law or tomaintain said flux at a constant value.

In accordance with the invention, the deuterons are produced in burstsby means of an ion source 3 equipped with a pulsation assembly which isidentical, for example, with the unit described in French Patent No.1,401,882 filed on July 22nd, 1964 in respect of Remote-control devicefor highfrequency pulsed ion source."

A clock 4 emits pulses having a variable repetition frequency. A controlcircuit 4' is triggered by the clock 4 and produces square waves havinga recurrence frequency which is defined by the value of the modulationor stabilization voltage the duration of said square waves is dependenton the value of the number of neutrons derived from the target 2. Adetail description of this modulated clock will be given hereinafter.

The output of the control circuit 4' is connected to animpedance-lowering differentiating circuit 5 and the signal obtained istransmitted by a coaxial cable 6 to the primary winding of a pulsetransformer 7, the secondary winding of which is at the potential of thehigh-voltage head. In order to reconstitute the control square-wavesignal which is differentiated by this assembly, the secondary windingdelivers the pulses into a flip-flop 8 followed by an amplifier 9, bothelements being placed within the high-voltage head. The output of theamplifier 9 serves to block or release the high-frequency oscillator 10.

Thus, within the time intervals in which said oscillator is released, anintense ionization takes place within the tube by reason of thehigh-frequency electric field between the two rings 13 and 14 thedeuteron delivery of the source is then at a maximum level and theemission of a neutron burst having the same duration as the deuteronpulse then takes place. On the contrary, within the time intervals inwhich the oscillator is blocked, no field is present within the tube andionization is zero there is therefore no further output from the sourceand no production of neutrons.

Opposite to the target 2, a semi-conductor junction 15 having an axiswhich is inclined substantially at an angle of 138 to the beam ofincident deuterons serves to count the a particles of 3 MeV energy whichare emitted at the time of the T (d,n) a reaction. The signals derivedfrom this junction are amplified, discriminated and shaped in a unit 16,then divided by a factor N by means of a decade divider 17 beforedriving the control circuit 4 at the same time as the clock pulses 4,the frequency of which is modulated or predetermined by a generator 18.

Said clock is illustrated in detail in FIG. 2. A differential amplifier19 constituting an impedance-lowereing device has two inputs connectedin one case to the generator 18 and in the other case to a variabledirect-current voltage source 20 which serves to regulate the meanlevel. The output of said amplifier controls a current generator whichis also constituted by a negative-feedback differential amplifier 21 andthis latter charges a capacitor 22, a unijunction transistor 23 beingconnected to the terminals of said capacitor. When the voltage developedacross the terminals of said capacitor attains the turn-on level of thetransistor 23, said transistor causes the capacitor to discharge andresults in the emission of a clock pulse, whereupon the cycle startsagain. Recurrent electric signals are thus obtained and the repetitionfrequency of these latter is a function of the voltage supplied by thegenerator 18 and the value of the capacitor 22. In order to makeprovision for a number of frequency ranges, the capacitor 22 isadvantageously made up of a plurality of capacitors which are associatedin parallel and can be switched over.

Said clock pulse which is collected at the base B1 of the unijunctiontransistor 23 is transmitted to the control circuit 4' constituted by abistable device formed by two transistors and causes said device tochange over from state to state I, thus providing the beginning of thesquare-wave signal which releases the high-frequency oscillator 10,thereby causing the ignition of the ion source 3. The pulses derivedfrom the detection unit are also transmitted to said bistable device viaa decade divider l7 and thus reset said device to state 0, therebyblocking the oscillator 10 and extinguishing the ion source. Thefollowing clock pulse again turns on the source and the same processbegins again.

Thus, the generator 18 serves either to modulate the frequency of thesource-ignition signals according to a given law and especially asinusoidal law or to maintain said frequency at a constant value, andthe duration of each of said square-wave signals is regulated independence on the measurement of the neutron production rate of thetarget 2. Any variation in said production rate other than that which isimposed by the generator 18 is thus immediately compensated either by anincrease or by a reduction in the duration of neutron bursts. However, acircuit which is not shown in FIG. 2 makes it possible to prevent theduration of said squarewave signals from being smaller than the timenecessary to ensure total ionization of the ion source, namelyapproximately 7 at. A device of this type serves to ensure sinusoidalmodulation or stabilization ofa neutron flux with a degree of accuracyofthc order ofa few percent.

An anthracene scintillator 25 placed in proximity to the tar get 2 andassociated with a photomultiplier 26 followed by a shaping thresholdunit 27 can also be employed for measuring the neutron production rateby detection of the recoil protons which are produced in saidscintillator.

A multichannel analyzer 28 which is employed as a multiscaler andconnected to the output of the detection unit makes it possible bystoring within the successive channels ofa memory device pulses whichappear during given time intervals to control the neutron productionrate of the target on the screen of its oscilloscope. Said analyzer isconnected to the generator 18 and receives from this latter asynchronization signal which initiates the analysis.

It will readily be understood that the present invention is not limitedsolely to the mode of execution which has been illustrated and describedby way of explanation and that the scope of this patent also extends toalternative forms of either all or part of the arrangements describedwhich remain within the definition of equivalent means as well as to anyapplications of such arrangements.

What we claim is:

1. A method of modulation or stabilization of a neutron flux obtained bybombardment of suitably charged target with nuclear particles comprisingproducing a beam of said particles in a pulsed form, modulating thepulsation frequency of said beam, measuring the neutron production rateof said target and controlling the duration of the pulsations of thesource in dependence on the measurement of the neutron production rateof said target.

2. A particle device for modulating or stabilizing a neutron fluxobtained by bombardment of a suitably charged target with neutronparticles wherein said device comprises:

a pulsed source of nuclear particles,

a neutron-generating target which receives said particles,

a detection unit placed in proximity to said target for the measurementof its neutron production rate,

a generator for producing a control voltage,

an electronic clock controlled by said generator for delivering pulsesat a variable frequency in accordance with said control voltage,

and an electronic control circuit connected to said source and receivingtrigger pulses from said electronic clock and blocking pulses from saiddetection unit for controlling the pulse frequency and the pulseduration of said source.

3. A device in accordance with claim 2, wherein said pulsed source isconstituted by a gas-filled chamber in which the ionization of said gasliberates said particles and which is fitted with electrodes forproducing a high-frequency electric field for the ionization of said gasand further constituted by a highfrequency oscillator which is connectedto said electrodes and which the release periods corresponding to theemission of particle bursts are controlled by the pulses delivered bysaid electronic control circuit.

4. A device in accordance with claim 2, wherein the detection unitcomprises a semi-conductor junction placed in front of the target andprotected from the beam of incident particles and a shaping unitassociated with said junction for counting a particles which are emittedat the time of the neutron production reaction.

5. A device in accordance with claim 2, wherein the detection unitcomprises an anthracene scintillator placed in proximity to the target,a photomultiplier associated with said scintillator and a unit forshaping the signals delivered by said photomultiplier for countingrecoil protons which are generated in the scintillator by the neutrons.

6. A device in accordance with claim 2, wherein said electronic clock isconstituted by:

a current generator connected to said generator for producing thecontrol voltage,

a capacitor connected to said current generator,

and a unijunction transistor connected to the terminals of saidcapacitor and causing the discharge of said capacitor when the terminalvoltage thereof attains the turn-on voltage of said transistor.

7. A device in accordance with claim 2, and further comprising a circuitfor preventing the duration of said pulses from being shorter than thetime which is necessary to ensure total ionization of the particlesource.

8. A device in accordance with claim 2, wherein said control voltageproduced by said generator is modulated voltage.

9. A device in accordance with claim 2, wherein the control voltageproduced by said generator is a continuous voltage.

1. A method of modulation or stabilization of a neutron flux obtained bybombardment of a suitably charged target with nuclear particlescomprising producing a beam of said particles in a pulsed form,modulating the pulsation frequency of said beam, measuring the neutronproduction rate of said target and controlling the duration of thepulsations of the source in dependence on the measurement of the neutronproduction rate of said target.
 2. A particle device for modulating orstabilizing a neutron flux obtained by bombardment of a suitably chargedtarget with neutron particles wherein said device comprises: a pulsedsource of nuclear particles, a neutron-generating target which receivessaid particles, a detection unit placed in proximity to said target forthe measurement of its neutron production rate, a generator forproducing a control voltage, an electronic clock controlled by saidgenerator for delivering pulses at a variable frequency in accordancewith said control voltage, and an electronic control circuit connectedto said source and receiving trigger pulses from said electronic clockand blocking pulses from said detection unit for controlling the pulsefrequency and the pulse duration of said source.
 3. A device inaccordance with claim 2, wherein said pulsed source is constituted by agas-filled chamber in which the ionization of said gas liberates saidparticles and which is fitted with electrodes for producing ahigh-frequency electric field for the ionization of said gas and furtherconstituted by a high-frequency oscillator which is connected to saidelectrodes and which the release periods corresponding to the emissionof particle bursts are controlled by the pulses delivered by saidelectronic control circuit.
 4. A device in accordance with claim 2,wherein the detection unit comprises a semi-conductor junction placed infront of the target and protected from the beam of incident particlesand a shaping unit associated with said junction for counting Alphaparticles which are emitted at the time of the neutron productionreaction.
 5. A device in accordance with claim 2, wherein the detectionunit comprises an anthracene scintillator placed in proximity to thetarget, a photomultiplier associated with said scintillator and a unitfor shaping the signals delivered by said photomultiplier for countingrecoil protons which are generated in the scintillator by the neutrons.6. A device in accordance with claim 2, wherein said electronic clock isconstituted by: a current generator connected to said generator forproducing the control voltage, a capacitor connected to said currentgenerator, and a unijunction transistor connected to the terminals ofsaid capacitor and causing the discharge of said capacitor when theterminal voltage thereof attains the turn-on voltage of said transistor.7. A device in accordance with claim 2, and further comprising a circuitfor preventing the duration of said pulses from being shorter than thetime which is necessary to ensure total ionization of the particlesource.
 8. A device in accordance with claim 2, wherein said controlvoltage produced by said generator is modulated voltage.
 9. A device inaccordance with claim 2, wherein the control voltage produced by saidgenerator is a continuous voltage.